For internal combustion engines that operate with more than one directly injected fuel, the fuels need to be delivered to one or more fuel injectors. The fuel injectors are configured either above or to the side of the cylinder head to directly introduce fuel into combustion chambers before or after intake valves close, and are situated under a valve cover that shields and isolates the top of the engine block from the outer environment. Fuel that is directly injected is typically supplied to a fuel rail in the form of a cylindrical tube (so called fabricated or assembled rails) that extends under the valve cover to deliver the fuel in close proximity to the direct fuel injectors. For each direct fuel injector, a fuel line of a branch connection extends directly between the fuel rail and the injector either above or to the side of the cylinder head. Alternatively, the fuel line can comprise a bore through the cylinder head to the side of the fuel injector where a fuel gallery extends around the fuel injector that is typically sealed with a radial seal. However, when one of the fuels is a gaseous fuel that undergoes long soak-in periods at high pressure the gallery seals can be compromised as gaseous fuel pressure drops rapidly, such as during engine shut-down events or when engine operating conditions change. When more than one fuel is directly injected, there needs to be at least one fuel rail for each fuel. Space constraints under the valve cover can make it difficult to package the fuel rails therein without modification to the engine compartment. One solution to this problem is to provide one or more internal rails to deliver one or both fuels through the cylinder head in the vicinity of the fuel injectors such that a branch connection can extend between the fuel rail and each fuel injector. However, for a variety of reasons the use of internal fuel rails is not always possible. The internal fuel rails can change the thermodynamic properties of the cylinder head that could lead to reliability and/or durability issues, and the fuels could be heated that although may provide ignition benefits, it may also lead to increased deposit formation in and around fuel injector nozzles.
Specially adapted couplings that employ a metal-to-metal sealing structure can connect the fuel lines to the cylindrical tube fuel rail. An end-form on the fuel line comprises a conical or spherical like shape that mutually engages an inwardly tapering section in an outer surface of the fuel rail. The inwardly tapering section is surrounded by a fitting, which can be welded to the fuel rail or the fitting can be an annular fitting that is slid into position along the fuel rail and fastened thereto. A nut threadedly engages the fitting to secure the end-form in place creating a fluidly sealed connection between the end-form and the inwardly tapering section. The compressive force between the end-form on the fuel line and the inwardly tapering section in a wall of the fuel rail puts constraints on the minimum thickness of the wall and/or the maximum fuel pressure that can be sealed. The specially adapted couplings can be excessively large, and the assembly of the cylindrical tube fuel rails with the specially adapted couplings and fuel lines can be complicated, labour intensive and time consuming.
The state of the art is lacking in techniques for delivering two or more fuels to each combustion chamber for one or more fuel injectors. The present apparatus and method provides a technique for improving the delivery of two or more fuels to direct fuel injectors in an internal combustion engine.